Method of laser annealing using linear beam having quasi-trapezoidal energy profile for increased depth of focus

ABSTRACT

A linear pulse laser beam to be applied to an illumination surface is so formed as to have, at the focus, an energy profile in the width direction which satisfies inequalities 0.5L 1≦ L 2≦ L 1  and 0.5L 1≦ L 3≦ L 1  where assuming that a maximum energy is 1, L 1  is a beam width of two points having an energy of 0.95 and L 1 + L 2 + L 3  is a beam width of two points having an energy of 0.70, L 2  and L 3  occupying two peripheral portions of the beam width. According to another aspect of the invention, a compound-eye-like fly-eye lens for expanding a pulse laser beam in a sectional manner is provided upstream of a cylindrical lens for converging the laser beam into a linear beam.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a technique of annealing, for instance,a semiconductor material by illuminating it with laser light. Theinvention generally relates to techniques of processing or modifying anobject in various manners by illuminating it with laser light.

The invention also relates to a laser annealing apparatus and method forannealing a semiconductor material by using a linear laser beam.

The invention is particularly effective when used, for instance, in aprocess of converting an amorphous silicon film into a crystallinesilicon film, a process of improving the crystallinity of a crystallinesilicon film, and a process of repairing lattice defects that have beengenerated by implanting an impurity into a crystalline silicon film to,for instance, render it conductive all of which processes are performedby laser annealing.

2. Description of the Related Art

In recent years, various studies have been made extensively to reducethe temperature of semiconductor device manufacturing processes. Themajor reason for this tendency is the need of forming semiconductordevices on an insulative substrate, such as a glass substrate, which isinexpensive and highly workable. Stated more specifically, this is dueto the need of forming thin-film transistors of several hundred byseveral hundred or more on a glass substrate in producing an activematrix liquid crystal display device. Other needs such as the needs offorming finer devices and multilayered devices have also prompted thestudies mentioned above.

In semiconductor manufacturing processes, it is sometimes necessary tocrystallize an amorphous semiconductor material or amorphous componentscontained in a semiconductor material, recover the crystallinity of asemiconductor material which was originally crystalline but has beenlowered in the degree of crystallinity due to ion irradiation forimpurity implantation, or improve the degree of crystallinity of analready crystalline semiconductor material. Conventionally, thermalannealing is used for these purposes. Where the semiconductor materialis silicon, crystallization of amorphous silicon, recovering orimprovement of crystallinity, etc. are attained by performing annealingat 600 to 1,100° C. for 0.1 to 48 hours or more.

In general, the above-mentioned thermal annealing may be performed in ashorter processing time when the temperature is higher. However, it hasalmost no effect when the temperature is 500° C. or less. Therefore,from the viewpoint of decreasing the temperature of a process, it isnecessary to replace a step that conventionally uses thermal annealingwith some other means.

In particular, where a glass substrate is used, it is required that thethermal annealing temperature be 700° C. or less, and that the heatingtime be as short as possible. The latter requirement is due to the factthat a long heat treatment may deform the glass substrate. In a liquidcrystal display device, a liquid crystal is held between a pair of glasssubstrates having a gap of several micrometers. Therefore, deformationof the glass substrates greatly affects display performance of theliquid crystal display device.

Various types of annealing technique using laser light illumination areknown as processes for replacing the thermal annealing. Laser light canimpart high energy that is equivalent to the energy obtained by thethermal annealing only to a desired portion; it is therefore notnecessary to expose the entire substrate to a high-temperatureatmosphere.

Stated in general, there have been proposed the following two laserlight illumination methods:

In the first method, a CW laser such as an argon ion laser is used and aspot-like beam is applied to a semiconductor material. A semiconductormaterial is crystallized such that it is melted and then solidifiedgradually due to a sloped energy profile of a beam and its movement.

In the second method, a pulsed oscillation laser such as an excimerlaser is used. A semiconductor material is crystallized such that it ismelted instantaneously by application of a high-energy laser pulse andthen solidified.

The first method has a problem of long processing time, because themaximum energy of a CW laser is insufficient and therefore the beam spotsize is at most several square millimeters. In contrast, the secondmethod can provide high mass-productivity, because the maximum energy ofa laser is very high and therefore the beam spot size can be madeseveral square centimeters or larger.

However, in the second method, to process a single, large-area substratewith an ordinary square or rectangular beam, the beam needs to be movedvertically and horizontally, which inconvenience still remains to besolved from the viewpoint of mass-productivity.

This aspect can be greatly improved by deforming a laser beam into alinear shape and moving the linear beam approximately perpendicularly toits longitudinal direction to effect scanning. The term “scanning” asused in this specification means illuminating an object while moving alinear laser beam step by step with an overlap in the beam widthdirection, that is, approximately perpendicularly to the longitudinaldirection of the beam.

The problem remaining unsolved is insufficient uniformity of laser lightillumination effects. The following measures have been taken to improvethe uniformity. A first measure is to make the beam profile as close toa rectangular one as possible by causing a laser beam to pass through aslit, to thereby reduce an energy variation within a linear beam.

FIGS. 4A and 4B show an energy profile of a laser beam; FIG. 4A shows anexample of a rectangular energy profile. The term “rectangular” as usedin this specification means a relationship L2, L3≦0.2L1 where L1 to L3are defined in FIG. 4B.

In using the above technique, it has been reported that the uniformitycan further be improved by performing preliminary illumination withweaker pulse laser light before illumination (hereinafter called “mainillumination”) with stronger pulse laser light.

This measure is so effective that the characteristics of resultingsemiconductor devices can be improved very much. This is because thetwo-step laser light illumination with different illumination energylevels allows a semiconductor film to be crystallized step by step,thereby reducing the seriousness of such problems as a non-uniformdistribution of crystallinity, formation of crystal grains, andconcentration of stress, which problems result from abrupt phasechanges.

The stepped illumination can be made more effective by increasing thenumber of illumination steps.

Thus, the above two kinds of measure can greatly improve the uniformityof the laser light illumination effects.

However, with the above two-step illumination method, the laserprocessing time is doubled, that is, the throughput is reduced.

Further, the equipment for the two-step illumination method is morecomplex than that for the single step illumination method, thus causinga cost increase.

In addition, although the above measures have much improved theuniformity of the laser light illumination effects, the degree ofimprovement is still insufficient.

To transform a square or rectangular light beam into a linear beam, aspecialized optical system is needed.

FIG. 14 shows an example of an optical system of a conventional laserannealing apparatus.

The optical system of FIG. 14 is composed of the following components.An excimer laser beam generating means A′ generates an excimer laserbeam. Beam expanders B′ and C′ expand the excimer laser beam. A verticalexpansion fly-eye lens D′ and a horizontal expansion fly-eye lens D2′expand the laser beam in a sectional manner. A first cylindrical lens E′converges the laser beam into a line shape. A second cylindrical lens F′improves the uniformity of the linear laser beam in its longitudinaldirection. A stage I′ is moved in direction J′ indicated by an arrow inFIG. 14 in a state that an illumination object, a substrate bearing anillumination object, or the like is placed thereon.

In FIG. 14, a path-folding mirror G′ and a cylindrical lens H′ serve toapply the laser beam to an object on the stage I′. In certain types ofconfiguration, the beam expanders B′ and C′ are omitted.

A uniform linear laser beam can be obtained by the above optical system.However, in this conventional optical system, the use of two fly-eyelenses for sectionally expanding a laser beam, that is, the fly-eye lensD′ for vertical expansion and the fly-eye lens D2′ for horizontalexpansion, lowers the transmittance of the entire fly-eye lens system,resulting in a low laser beam energy efficiency. As a result, in laserannealing, the amount of energy applied to an illumination object may belowered, possibly making the annealing insufficient.

To prevent this problem, the output of the laser light source needs tobe increased. But this increases the load on the laser light source, sothat the life of the entire apparatus may be shortened.

SUMMARY OF THE INVENTION

In view of the above, a first object of the present invention is toobtain highly uniform laser light illumination effects in crystallizinga semiconductor coating by using a linear laser beam emitted from apulsed laser.

In particular, it is an object of the invention to obtain highly uniformlaser light illumination effects by single step illumination, that is,without using a two-step scheme consisting of preliminary illuminationand main illumination.

A second object of the invention is to provide a laser annealingapparatus and method which are intended to generate a uniform linearlaser beam for use in laser annealing particularly to crystallize anamorphous silicon film formed on an insulative substrate such as a glasssubstrate, or improve the crystallinity of a thermally crystallizedsilicon film formed on an insulative substrate such as a glasssubstrate, and in which apparatus and method an optical system used islow in energy loss and capable of applying sufficient energy to anillumination object, and a laser light source has a long life.

The invention attains the first object by properly adjusting the energyprofile of a linear laser beam. More specifically, the invention causesa linear laser beam to have, at its focus, a quasi-trapezoidal energy(density) profile in its width direction (i.e., laser beam scanningdirection).

Processing such as crystallization is performed by applying a laser beamhaving the above energy profile to a semiconductor material coatingwhile scanning the coating with the laser beam.

Major aspects of the invention will be described below.

According to one of the major aspects of the invention, there isprovided a laser annealing method in which a linear laser beam emittedfrom a pulsed laser light source is applied to an illumination surfacethat is a semiconductor coating, wherein:

the linear laser beam has, at a focus, an energy profile in a widthdirection thereof which satisfies inequalities 0.5L1≦L2≦L1 and0.5L1≦L3≦L1 where assuming that a maximum energy is 1, L1 is a beamwidth of two points having an energy of 0.95 and L1+L2+L3 is a beamwidth of two points having an energy of 0.70, L2 and L3 occupying twoperipheral portions of the beam width.

According to another aspect of the invention, there is provided a laserannealing method in which a linear laser beam emitted from a pulsedlaser light source is applied plural times to an illumination surfacethat is a semiconductor coating while the linear laser beam and theillumination surface are moved relative to each other in a widthdirection of the linear laser beam, wherein:

the linear laser beam has, at a focus, an energy profile in a widthdirection thereof which satisfies inequalities 0.5L1≦L2≦L1 and0.5L1≦L3≦L1 where assuming that a maximum energy is 1, L1 is a beamwidth of two points having an energy of 0.95 and L1+L2+L3 is a beamwidth of two points having an energy of 0.70, L2 and L3 occupying twoperipheral portions of the beam width.

FIG. 5 illustrates how a linear laser beam having a quasi-trapezoidalenergy profile is applied.

Referring to FIG. 5, pulse laser beams having an energy density profileas shown in FIG. 4B (the beam width is defined as a half width of amaximum energy value of a laser beam) are applied while being movedgradually with overlaps. In this case, a linear region at a particularlocation is illuminated with plural pulses. During this illuminationwith plural pulses, the illumination energy density of pulses increasesin a step-like manner at the first stage and then decreases also in astep-like manner.

That is, the invention is characterized in that in applying linear pulselaser beams while moving those in one direction, they are applied in anoverlapped manner so that an arbitrary point on an illumination objectis illuminated with pulse laser beams plural times, that is, 3 to 100times, preferably 10 to 40 times.

In the above aspects of the invention, which are intended to attain thefirst object, in applying linear pulse laser beams while moving those intheir width direction, they are given a quasi-trapezoidal energy profilein the width direction.

The quasi-trapezoidal energy profile means a profile that satisfiesinequalities 0.5L1≦L2≦L1 and 0.5L1≦L3≦L1 where assuming that a maximumenergy is 1, L1 is a beam width of two points having an energy of 0.95and L1+L2+L3 is a beam width of two points having an energy of 0.70, L2and L3 occupying two peripheral portions of the beam width.

When linear laser beams having the above energy profile are appliedwhile moving those, an arbitrary point in an illumination region isfirst illuminated with low-energy-density laser beams corresponding tothe bottom portion of the trapezoidal energy profile. As laser beams aremoved subsequently, the energy density gradually increases, and laserbeams having an energy density corresponding to the top base (having amaximum value) of the trapezoidal energy profile come to be applied.Finally, the energy density gradually decreases.

In this manner, an arbitrary point in the illumination region isilluminated with laser beams whose energy density varies continuously soas to correspond to the trapezoidal energy profile.

Therefore, the bottom portions having an energy gradient of the abovetrapezoidal energy profile substantially has the role of the preliminaryillumination of weak laser light energy of the above-mentioned two-steplaser light illumination that consists of the preliminary illuminationand the main illumination. Thus, the invention can provide the sameeffects as in the case of changing the illumination energy in astep-like manner.

That is, a situation equivalent to the situation in which an arbitrarypoint in an illumination region is first illuminated with weak laserbeams, then laser beams whose intensity is gradually increased and thenlaser beams whose intensity is gradually decreased, and the illuminationis finished can be realized by applying laser beams in theabove-described manner rather than using the two-step illumination.

With the above laser light illumination, since the energy supplied to anillumination region does not vary abruptly, abrupt phase changes etc.can be prevented from occurring in the illumination object.

Therefore, for instance in crystallizing an amorphous semiconductor byilluminating it with laser light, by virtue of the absence of abruptphase changes, there does not occur surface roughening or accumulationof internal stress, enabling a uniform distribution of crystallinity,that is, uniform annealing effects.

Further, the illumination with the trapezoidal energy profile makes thedepth of focus of a laser beam wider than that of a conventional laserbeam, thereby facilitating laser processing.

In contrast to the fact that a conventional laser beam having therectangular energy profile has a depth of focus of about ±200 μm, alaser beam having the trapezoidal energy profile that satisfies0.5L1≦L2≦L1 and 0.5L1≦L3≦L1 provides a depth of focus of about ±400 μm.

FIG. 7 schematically shows a relationship between the laser beam energyprofile and the depth of focus (absolute value). A hatched region bcorresponds to the laser beam energy profile of the invention whichsatisfies 0.5L1≦L2≦L1 and 0.5L1≦L3≦L1. The horizontal axis representsL2/L1 (or L3/L1). As this value approaches 0, the laser beam energyprofile becomes closer to a rectangle. Conversely, as this value becomeslarger, the energy profile comes to assume a trapezoid or triangle.

A wide depth of focus of a laser beam allows laser processing to beperformed uniformly even on an illumination surface having a certaindegree of undulation or asperities.

For example, after a 0.2-μm-thick silicon oxide film and a 0.1-μm-thickamorphous silicon film are sequentially deposited on a glass substrateand thermal crystallization is performed at 600° C., the glass substrateis likely to have undulation of plus and minus several tens ofmicrometers to several hundred micrometers if it is about 300×300 mm² insize.

In such a case, a laser beam having the conventional rectangular profileof region a of FIG. 7 (0.5L1>L2, L3) has a depth of focus of about ±200μm, non-uniform crystallization occurs in the amorphous silicon film. Asa result, a crystallized silicon film likely has a mobility variation aslarge as more than 10% in the substrate area.

In contrast, in region c (L2, L3>L1), where the depth of focus is toowide, the focus adjustment becomes difficult and the energy densityimparted to an illumination object becomes too low. As a result, thecrystallization of the amorphous silicon film becomes insufficient and adesired mobility is not obtained.

A laser beam produced in the above manner has a depth of focus of about±400 μm, and therefore is more resistant, by a factor of about 2 to 8,to undulation of a substrate or coating than a conventional laser beam.Thus, laser processing on a silicon film having asperities of the abovekind can be performed very uniformly at a sufficiently high energydensity.

Therefore, even a silicon film formed on a substrate having undulationof several hundred micrometers can be processed to have a uniformmobility distribution having a variation of less than 10% andsufficiently large mobility values.

As such, a laser beam having the trapezoidal energy profile of theinvention can provide very uniform laser light illumination to even anillumination surface, such as a semiconductor coating, having undulationor asperities.

The above effects become more effective as the substrate size becomeslarger.

Since the above aspects of the invention provides a depth of focus ofabout ±400 μm, it enables uniform crystallization on an illuminationobject whose asperities are less than about ±400 μm.

With the depth of focus of the above level, in crystallizing a siliconcoating by laser light illumination, the crystallization can be made souniform that a mobility variation of the coating falls within ±10%.

It is noted that the above values are ones obtained in a case where ashot-by-shot energy variation of pulse laser beams falls within ±3% interms of 3σ. Where pulse laser beams have an energy variation that isequal to or larger than ±3% in terms of 3σ, the depth of focus isreduced. Pulse laser beams having an energy variation that is equal toor larger than ±10% in terms of 3σ are not suitable for crystallizationof a semiconductor.

To attain the second object, according to another aspect of theinvention, there is provided a laser annealing apparatus comprising (seeFIG. 9):

pulse laser beam generating means (K) for generating a pulse laser beam;

beam expanders (L, M) for expanding the generated laser beam;

a compound-eye-like fly-eye lens (N) for expanding, sectionally, theexpanded laser beam;

a first cylindrical lens (O) for converging the sectionally expandedlaser beam into a linear laser beam;

a second cylindrical lens (P) for improving uniformity of the linearlaser beam in a longitudinal direction thereof; and

a stage (S) for moving an illumination object relative to the linearlaser beam approximately perpendicularly to the longitudinal directionthereof.

According to a still another aspect of the invention, there is provideda laser annealing apparatus comprising (see FIG. 10):

pulse laser beam generating means (k) for generating a pulse laser beam;

a compound-eye-like fly-eye lens (l) for expanding, sectionally, thepulse laser beam;

a first cylindrical lens (m) for converging the sectionally expandedlaser beam into a linear laser beam;

a second cylindrical lens (n) for improving uniformity of the linearlaser beam in a longitudinal direction thereof; and

a stage (q) for moving an illumination object relative to the linearlaser beam approximately perpendicularly to the longitudinal directionthereof.

In the above configurations, it is preferred that the pulse laser beamgenerating means be excimer laser beam generating means.

It is preferred that a slit for eliminating a peripheral portion of thelinear laser beam be provided downstream of the first cylindrical lens.

It is also preferred that the compound-eye-like fly-eye lens beconfigured such that a plurality of convex lenses each having apolygonal sectional shape are arranged regularly and adjacently into aplanar shape. It is preferred that each convex lens has a square,rectangular, hexagonal, or like sectional shape.

To attain the second object, according to a further aspect of theinvention, there is provided a laser annealing method comprising thesteps of:

expanding, sectionally, a pulse laser beam with a compound-eye-likefly-eye lens;

converging the sectionally expanded laser beam into a linear laser beam;and

illuminating and scanning an illumination object with the linear laserbeam.

In the above method, it is preferred that the pulse laser beam be anexcimer laser beam.

In the above laser annealing apparatus and method, a laser beam asgenerated by the excimer laser beam generating means or a laser beamthus generated and then expanded and shaped by the beam expanders isexpanded, in a sectional manner, by the single compound-eye-like fly-eyelens.

With this configuration, the loss of light quantity is reduced, comparedwith the case of using two fly-eye lenses for vertical expansion andhorizontal expansion. As a result, the loss of the laser beam energy isgreatly reduced, that is, the energy efficiency is improved. This makesit possible to provide superior laser annealing and crystallization of asilicon film, and to elongate the life of a laser light source.

FIG. 8 shows an example of the compound-eye-like fly-eye lens. Thecompound-eye-like fly-eye lens of the invention is constructed byarranging, regularly and adjacently, a plurality of convex lenses 801each having a polygonal, for instance, square, sectional shape into aplanar shape. This compound-eye-like fly-eye lens has a function ofuniformly expanding, in a sectional manner, incident light in bothvertically and horizontally, though it is a single lens.

It is preferable that the individual convex lenses constituting thefly-eye lens assume a polygon, in particular, a rectangle, square,hexagon, or the like. This is because in such a case they can easily bearranged regularly, and hence the fly-eye lens can be formed and workedeasily. Further, the fly-eye lens can easily be given high precision.

The lenses that were mentioned above as the components of the laserannealing apparatus serve to converge a laser beam into a linear beamand to make the beam energy profile uniform in the width direction.After a laser beam is expanded by the beam expanders and/or the fly-eyelens, it is converged into a linear beam by a rod-shaped converging lensthat is cylindrical in one direction, for instance, a cylindrical lens.

Immediately after emission, an excimer laser beam as a pulse laser beamhas a rectangular cross-section and a generally uniform intensitydistribution in the cross-section.

The beam expanders increases the width of the laser beam, and expandsand shapes the beam cross-section into a square (or rectangular) shape,thus increasing the cross-sectional area.

However, the use of the beam expanders reduces the energy efficiency asmuch as the increase in the number of lenses. Therefore, the beamexpanders may be omitted.

In addition to expanding the beam area, the fly-eye lens has a functionof making the beam energy profile uniform. It is noted that originallythe fly-eye lens was developed to provide a uniform beam.

Due to the spherical aberration, a laser beam as converged into a linearshape has an energy profile in the width direction which includes lowenergy density portions at beam peripheries like those of a Gaussiandistribution. Therefore, the peripheral portions of a linear laser beamdo not end in a definite manner.

In view of the above, a proper slit may be used to cut the peripheralportions (bottom portions) of the Gaussian-distribution-like energyprofile in the width direction which portions occur in the linear laserbeam after passage through the cylindrical lens.

Laser beams are applied, with proper overlaps, to an illumination objectsuch as an amorphous silicon film formed on a glass substrate whichobject is placed on a stage while the stage is moved. In this manner,the amorphous silicon film and the like can be crystallized uniformly athigh speed.

The laser annealing apparatus and method of the invention areparticularly effective when they are applied to, for example, a step ofconverting an amorphous silicon film into a crystalline silicon film bylaser annealing, a step of improving the crystallinity of a crystallinesilicon film, and a step of repairing lattice defects that occur whenimpurity ions have been implanted into a crystalline silicon film to,for instance, render it conductive.

In particular, the above apparatus and method are effectively applied tovarious kinds of film formed on a glass substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the concept of a laser annealing apparatus used inembodiments of the present invention;

FIG. 2 shows an example of a laser annealing optical system used in afirst embodiment of the invention;

FIG. 3 shows an example of a laser annealing optical system used in asecond embodiment of the invention;

FIGS. 4 a and 4 b show an energy profile of a laser beam;

FIG. 5 illustrates an energy density profile of a linear laser beam inits width direction (scanning direction);

FIGS. 6A to 6F show a manufacturing process of a thin-film transistoraccording to the second embodiment of the invention;

FIG. 7 schematically shows a relationship between the laser beam energyprofile and the depth of focus;

FIG. 8 shows an example of a compound-eye-like fly-eye lens;

FIG. 9 shows an example of a laser annealing optical system used in athird embodiment of the invention;

FIG. 10 shows an example of a laser annealing optical system used in afourth embodiment of the invention;

FIG. 11 shows the operation of a cylindrical lens;

FIG. 12 illustrates how laser light illumination is performed by using alinear laser beam;

FIGS. 13A to 13F show a manufacturing process of a thin-film transistoraccording to third and fourth embodiments of the invention; and

FIG. 14 shows an example of a laser annealing optical system of aconventional laser annealing apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1

In this embodiment, a silicon film is used as a semiconductor material.A description will be made of a technique of improving the crystallinityof a silicon film by illuminating it with laser light.

First, an apparatus will be described.

FIG. 1 shows the concept of a laser annealing apparatus used in thisembodiment, which is of a multi-chamber type. Each substrate is inputthrough a loader/unloader chamber 11, and then properly positioned in analignment chamber 12. The substrate is then sequentially transferred torespective chambers via a transfer chamber 13 by means of a substratetransfer robot 14 that is provided in the transfer chamber 13, andprocessed in the respective chambers.

That is, a substrate is first input to a heat treatment chamber 15.After being subjected to a heat treatment, the substrate is subjected tolaser annealing in a laser annealing chamber 16, cooled in a slowcooling chamber 17, and then moved to the loader/unloader chamber 11.

Being airtight, this laser annealing apparatus is free of pollution byimpurities. This apparatus has a function of controlling an atmosphereduring laser light illumination. Further, this apparatus has a functionof heating a substrate, whereby an object can be kept at a desiredtemperature during laser light illumination.

In this laser annealing apparatus, an energy variation from one pulse toanother falls within ±3% in terms of 3σ. Although a pulsed laser havinga larger energy variation than the above range may be used, the depth offocus becomes shorter in such a case. Laser annealing apparatuses withan energy variation larger than ±10% in terms of 3σ are not suitable foruse in this embodiment.

The laser annealing apparatus of this embodiment is provided with alaser beam emitting means (not shown). A linear beam emitted from thelaser beam emitting means is input to the laser annealing chamber 16,and applied to a sample that is placed on a stage of the laser annealingchamber 16.

An oscillator of the laser beam emitting means is type EX 748 of LumnicsCorp., which generates KrF excimer laser light (wavelength: 248 nm;pulse width: 25 ns).

Naturally, other excimer lasers and other types of laser can be used aslong as they are of a pulsed oscillation type.

An emitted laser beam is input to an optical system shown in FIG. 2 totransform its shape.

Immediately before entering the optical system, the laser beam assumes arectangle of about 3×2 cm². It is converted by the optical system into along and narrow beam (linear beam) of 10 to 30 cm in length and 0.01 to0.3 cm in width.

After passing through the optical system, the linear laser beam in thewidth direction assumes a trapezoidal energy density profile as shown inFIG. 4B, and has a maximum energy of 800 mJ/shot.

The reason for converting the emitted laser beam into a long and narrowbeam is to improve its workability. That is, in illuminating a samplewith a linear laser beam, the entire sample can be illuminated by movingthe sample in one direction if the beam is longer than the sample width.

Even if the beam is shorter than the sample width, the processing willbe easier than in the case of a rectangular beam. However, in this case,it is necessary to move the beam in vertically and horizontally relativeto a sample.

A substrate (sample) to be illuminated with a laser beam is placed onthe stage that is provided in the laser annealing chamber 16. The stageis controlled by a computer, and so designed as to move perpendicularlyto the longitudinal direction of the linear laser beam.

If the stage is provided with an additional function of moving in thelongitudinal direction of the beam, even a sample that is wider than thebeam length can be laser-processed in its entirety. Further, since thestage incorporates a heater in its lower portion, a sample can be keptat a given temperature during laser light illumination.

Next, referring to FIG. 2, a description will be made of an optical pathin an optical system for converting a laser beam into a linear beam.

A laser beam that has been input to the optical system passes through acylindrical concave lens B, a cylindrical convex lens C (the lenses Band C are together called a beam expander), and a compound-eye-likefly-eye lens D. The compound-eye-like fly-eye lens D may be replaced bytwo fly-eye lenses D and D2 for vertical and horizontal expansion. Adetailed description of the compound-eye-like fly-eye lens will be madein the third embodiment of the invention.

Thereafter, the laser beam passes through a cylindrical convex lens E(first cylindrical lens) and a cylindrical convex lens F (secondcylindrical lens) for improving the uniformity in the longitudinaldirection of a resulting linear beam, reflected by a mirror G, convergedby a cylindrical lens H, and finally applied to an illumination surface.

The interval between the cylindrical lenses A and B, the intervalbetween the fly-eye lenses D and D2, the interval between the fly-eyelens D and the cylindrical lens E, and the interval between thecylindrical lens F and the illumination surface are set at 230 mm, 230mm, 650 mm, and 650 mm, respectively (each interval is equal to a sum offocal lengths of the respective lenses concerned). Apparently theseintervals may be changed as occasion demands. The cylindrical lens H hasa focal length 120 mm.

The energy profile of the laser beam at the focus is renderedtrapezoidal by moving the lens H vertically (i.e., in direction J).

By moving the illumination surface vertically (i.e., in direction J)relative to the lens H, the energy profile of the laser beam on theillumination surface (i.e., at the focus) can be changed from arectangle-like one to a trapezoid-like one, as shown in the bottom partof FIG. 2. The energy profile can be made sharper by inserting a slit inthe laser beam path.

Any optical system may be used as long as it can transform a laser beaminto a beam having a shape that is required by the invention.

The optical system is not limited to the one shown in FIG. 2; there maybe used an optical system shown in FIG. 3 which does not include thelenses B and C.

Next, a description will be made of an example of forming a crystallinesilicon film on a glass substrate by using laser light illuminationaccording to the invention.

First, a square glass substrate (for instance, Corning 7059 or 1737) of30 cm by 30 cm is prepared.

A 2,000-Å-thick silicon oxide film is formed on the glass substrate bythe plasma CVD by using TEOS as a material. This silicon oxide filmserves as an undercoat film for preventing impurities from diffusingfrom the glass substrate side into a semiconductor film.

Next, an amorphous silicon film is formed by the plasma CVD. Thelow-pressure thermal CVD may be used instead of the plasma CVD. Thethickness of the amorphous silicon film is set at 500 Å in thisembodiment, but naturally it may be set at a desired value.

Then, hydrogen is removed from the amorphous silicon film by keeping theabove structure at 450° C. for 1 hour in a nitrogen atmosphere. This isto reduce a threshold energy in a subsequent crystallization step byintentionally forming dangling bonds in the amorphous silicon film.

Thereafter, a metal element for accelerating crystallization of siliconis introduced. In this embodiment, nickel is used as the metal element.To introduce the nickel element, a nickel acetate salt solution isapplied to the amorphous silicon film so that the nickel element is heldin contact with the surface of the amorphous silicon film. The amorphoussilicon film is crystallized by performing a heat treatment of 550° C.and 4 hours in a nitrogen atmosphere.

Thus, a crystalline silicon film is obtained on the glass substrate.However, the crystalline silicon film obtained in this manner containsmany amorphous components in the inside. The crystalline silicon film inthis state may cause deteriorations or variations of electricalcharacteristics. To avoid this problem, in this embodiment, thecrystallinity is improved by performing laser light illumination inaddition to the crystallization by the above heat treatment.

In this state, the glass substrate and the silicon film formed thereonhad asperities of about ±200 μm.

In this embodiment, KrF excimer laser light (wavelength: 248 nm; pulsewidth: 25 ns) is applied to the crystalline silicon film by using theapparatus of FIG. 1. The crystallinity can be improved by this laserlight illumination.

A laser beam is shaped into a linear beam so as to have a beam area of125 mm×1 mm on the illumination surface. The beam width is defined as ahalf width of a maximum value of the laser beam energy.

The energy profile of the linear laser beam in its width direction isquasi-trapezoidal and has dimensions L1=0.4 mm and L2=L3=0.25 mm (seeFIG. 4B). These dimensions satisfy inequalities 0.5L1≦L2≦L1 and0.5L1≦L3≦L1.

The degree of expanse of the bottom portions of this trapezoidal profiledepends on the distance between the final lens of the laser opticalsystem and the illumination surface. During laser light processing, thedistance between the final lens of the laser optical system and theillumination surface varies due to asperities of an illumination object.The degree of expanse of the bottom portions of the laser beamtrapezoidal profile varies accordingly. However, if the variation rangefalls within the ranges of the above inequalities, uniform laser lightprocessing can be performed. The term “uniform” as used herein meansthat the mobility variation of the laser-light-illuminated film in thesubstrate area falls within ±10%.

A sample is placed on the stage of the laser annealing chamber 16 (seeFIG. 1), and illumination is effected while the stage is moved at 2mm/s. The laser light illumination conditions are set such that thelaser light energy density is 100 to 500 mJ/cm² (300 mJ/cm² in thisembodiment) and the pulse rate is 30 pulses/s. The term “energy density”as used herein means a density of the top base portion (which has amaximum value) of a trapezoidal beam profile.

If laser light illumination is effected under the above conditions, anarbitrary point on the sample is subjected to 15-step illumination. Thatis, since the beam takes 0.5 second to pass through an arbitrary point,that point is illuminated with 15 beam pulses during one scan. Among 15times of illumination, the illumination energy density graduallyincreases in first several times of illumination and gradually decreasesin last several times of illumination.

FIG. 5 schematically illustrates this illuminating operation. The laserlight energy gradually increases in the first half of the 15 steps(indicated by character A in FIG. 5) and gradually decreases in the lasthalf (indicated by character B in FIG. 5). The number 15 can easily becalculated from the laser beam width, the moving speed of the stage, andthe laser pulse rate.

According to our experiments, silicon films having highest degrees ofcrystallinity were obtained by 3 to 100 steps of illumination,preferably 10 to 40 steps of illumination.

To reduce the rates of increase and decrease of the substrate surfacetemperature due to laser light illumination, the substrate temperatureis kept at 500° C. during the laser light illumination. It is known thatin general an abrupt change in environmental conditions impairs theuniformity of a substance. A degradation in the uniformity of thesubstrate surface due to the laser light illumination is minimized inthis embodiment by keeping the substrate temperature high. Although thesubstrate temperature is set at 500° C. in this embodiment, inpracticing the invention it is set at a temperature suitable for laserannealing in a range from the room temperature to the strain point ofthe glass substrate.

No specific atmosphere control is performed in this embodiment, that is,the illumination is performed in the air. Alternatively, theillumination may be performed in a vacuum, in an atmosphere of an inertgas such as argon or helium, or in an atmosphere of hydrogen, nitrogen,or the like.

In this embodiment, although illumination objects had asperities ofabout ±200 μm, in crystallized coatings a mobility variation in thesubstrate area was as low as about 7%, which indicates that the laserprocessing was performed uniformly.

On the other hand, another experiment was conducted in which the energyprofile of a linear laser beam in the width direction was made aquasi-trapezoidal shape that is somewhat close to a rectangular, thatis, L1=0.5 mm and L2=L3=0.2 mm, which satisfy 0.5L1>L2=L3 (see FIG. 4B).When illumination objects (silicon films) having asperities of the abovelevel were subjected to the laser annealing, a mobility variation was±13%.

A still another experiment was conducted in which the energy profile ofa linear laser beam in the width direction was made a quasi-trapezoidalshape that is somewhat close to a rectangular, that is, L1=0.2 mm andL2=L3=0.3 mm, which satisfy L1>L2=L3 (see FIG. 4B). When illuminationobjects (silicon films) having asperities of the above level weresubjected to the laser annealing, a mobility variation was 8%. However,mobility values were very small for crystalline silicon films.

Embodiment 2

This embodiment is directed to a case where a plurality of island-likepatterned regions that are amorphous silicon films, on a glass substrateare converted by laser annealing into substantially single-crystallinesilicon films, which are used as active layers of thin-film transistors.

As in the case of the first embodiment, the laser annealing apparatus ofFIG. 1 is used in this embodiment.

An oscillator of type 3000-308 produced by Lambda Physic Corp. is used,which emits XeCl excimer laser light (wavelength: 308 nm; pulse width 26ns). Naturally, other excimer lasers and other types of laser can beused as long as they are of a pulsed oscillation type.

To transform the shape of an emitted laser beam, it is input to anoptical system as shown in FIG. 3.

A laser beam, which assumes a rectangle of about 3×2 cm² immediatelybefore entering the optical system, is shaped into a long and narrowbeam (i.e., linear beam) of 10 to 30 cm in length and 0.01 to 0.3 cm inwidth by the optical system.

The linear laser beam as output from the optical system has, in thewidth direction, a trapezoidal energy density profile as shown in FIG.4B, and also has a maximum energy of 1,000 mJ/shot.

The reason for converting the emitted laser beam into a long and narrowbeam is to improve its workability. That is, in illuminating a samplewith a linear laser beam, the entire sample can be illuminated by movingthe sample in one direction if the beam is longer than the sample width.

Even if the beam is shorter than the sample width, the processing willbe easier than in the case of a rectangular beam. However, in this case,it is necessary to move the beam in vertically and horizontally relativeto a sample.

The stage on which an illumination subject substrate (sample) is to beplaced is controlled by a computer, and so designed as to moveperpendicularly to the longitudinal direction of the linear laser beam.

If the stage is provided with an additional function of moving in thelongitudinal direction of the beam, even a sample that is wider than thebeam length can be laser-processed in its entirety. Further, since thestage incorporates a heater in its lower portion, a sample can be keptat a given temperature during laser light illumination.

Next, referring to FIG. 3, a description will be made of an optical pathin an optical system for converting a laser beam into a linear beam.

First, a laser beam that has been emitted from a laser light source aand input to the optical system passes through fly-eye lenses b and cfor vertical expansion and horizontal expansion, respectively.

Thereafter, the laser beam passes through a cylindrical convex lens d(first cylindrical lens) and a cylindrical convex lens e (secondcylindrical lens) for improving the uniformity in the longitudinaldirection of a resulting linear beam, reflected by a mirror f, convergedby a cylindrical lens g, and finally applied to the sample.

As for optical path lengths, the distance between the laser light sourceand the mirror g is 2,000 mm, and the distance between the mirror g andthe illumination surface is 440 mm. The cylindrical lens g has a focallength 100 mm.

The energy profile of the laser beam at the focus is renderedtrapezoidal by moving the lens g vertically (i.e., in direction i).

By moving the illumination surface vertically (i.e., in direction i)relative to the lens g, the energy profile of the laser beam on theillumination surface (i.e., at the focus) can be changed from arectangle-like one to a trapezoid-like one, as shown in the bottom partof FIG. 2. The energy profile can be made sharper by inserting a slit inthe laser beam path.

Any optical system may be used as long as it can transform a laser beaminto a beam having a shape that is required by the invention.

The optical system is not limited to the one shown in FIG. 3; there maybe used an optical system shown in FIG. 2 which includes the lenses Band C.

Next, with reference to FIGS. 6A to 6F, a description will be made of amanufacturing process in accordance with this embodiment.

First, a square glass substrate (for instance, Corning 7059 or 1737) of30 cm by 30 cm is prepared.

A 2,000-Å-thick silicon oxide film 602 is formed on the glass substrateby the plasma CVD by using TEOS as a material. This silicon oxide film602 serves as an undercoat film for preventing impurities from diffusingfrom the glass substrate side into a semiconductor film.

Next, an amorphous silicon film 603 is formed by the plasma CVD.Low-pressure thermal CVD may be used instead of the plasma CVD. Thethickness of the amorphous silicon film 603 is set at 500 Å in thisembodiment, but naturally it may be set at a desired value. (FIG. 6A)

Then, a metal element for accelerating crystallization of silicon isintroduced. In this embodiment, nickel is used as the metal element. Tointroduce the nickel element, a nickel acetate salt solution is appliedto the amorphous silicon film so that the nickel element is held incontact with the surface of the amorphous silicon film.

Thereafter, a plurality of island-like regions are formed on the glasssubstrate by patterning the amorphous silicon film 603 so that eachisland-like region assumes a square with each side being several tens ofmicrometers to several hundred micrometers, and 90 μm in thisembodiment. The respective island-like regions are located at positionsof a plurality of thin-film transistors later formed, and now constituteactive layers 604 of those thin-film transistors. In this state, theactive layer 604 is an amorphous silicon film. (FIG. 6B)

In this state, the active layer 604 is crystallized by illuminating itwith XeCl excimer laser light (wavelength: 308 nm; pulse width: 25 ns)by using the apparatus of FIG. 1.

A laser beam is shaped into a linear beam so as to have a beam area of150 mm×0.4 mm on the illumination surface. The beam width is defined asa half width of a maximum value of the laser beam energy.

The energy profile of the linear laser beam in its width direction isquasi-trapezoidal and has dimensions L1=0.1 mm and L2=L3=0.08 mm (seeFIG. 4B). These dimensions satisfy inequalities 0.5L1≦L2≦L1 and0.5L1≦L3≦L1.

The degree of expanse of the bottom portions of this trapezoidal profiledepends on the distance between the final lens of the laser opticalsystem and the illumination surface. During laser processing, thedistance between the final lens of the laser optical system and theillumination surface varies due to asperities of an illumination object.The degree of expanse of the bottom portions of the laser beamtrapezoidal profile varies accordingly. However, if the variation rangefalls within the ranges of the above inequalities, uniform laser lightprocessing can be performed. The term “uniform” as used herein meansthat the mobility variation of the laser-light-illuminated film fallswithin ±10%.

The glass substrate 601 is placed on the stage, and illumination iseffected while the stage is moved at 2.5 mm/s. In the laser lightillumination, the active layer 604 is scanned with a linear laser beamfrom its one side to the opposite side.

The laser light illumination conditions are set such that the laserlight energy density is 100 to 500 mJ/cm² (400 mJ/cm² in thisembodiment) and the pulse rate is 200 pulses/s. The term “energydensity” as used herein means a density of the top base portion (whichhas a maximum value) of a trapezoidal laser beam energy profile.

If laser light illumination is effected under the above conditions, anarbitrary point on the sample is subjected to 32-step illumination. Thatis, since the beam takes 0.4 second to pass through an arbitrary point,that point is illuminated with 32 beam pulses during one scan. Among 32times of illumination, the illumination energy density graduallyincreases in first several times of illumination and gradually decreasesin last several times of illumination.

FIG. 5 schematically illustrates this illuminating operation. The laserlight energy gradually increases in the first half of the 32 steps(indicated by character A in FIG. 5) and gradually decreases in the lasthalf (indicated by character B in FIG. 5). The number 32 can easily becalculated from the laser beam width, the moving speed of the stage, andthe laser pulse rate.

To reduce the rates of increase and decrease of the substrate surfacetemperature due to laser light illumination, the substrate temperatureis kept at 500° C. during the laser light illumination. It is known thatin general an abrupt change in environmental conditions impairs theuniformity of a substance. A degradation in the uniformity of thesubstrate surface due to the laser light illumination is minimized inthis embodiment by keeping the substrate temperature high. Although thesubstrate temperature is set at 500° C. in this embodiment, inpracticing the invention it is set at a temperature suitable for laserannealing in a range from the room temperature to the strain point ofthe glass substrate.

No specific atmosphere control is performed in the embodiment, that is,the illumination is performed in the air. Alternatively, theillumination may be performed in a vacuum, in an atmosphere of an inertgas such as argon or helium, or in an atmosphere of hydrogen, nitrogen,or the like.

When a linear laser beam is applied to the active layer 604 that is anamorphous silicon film, an illuminated portion is meltedinstantaneously. As the active layer 604 is illuminated while beingscanned, crystal growth gradually proceeds therein, whereby a regionthat can substantially be considered a single crystal is produced.

That is, as the active layer 604 that is an amorphous silicon film isilluminated with a linear light beam while being scanned therewithgradually from its one end (see FIG. 6C), a portion 605 that cansubstantially be considered a single crystal grows. Finally, the entireactive layer 604 is rendered into a state that can be considered asingle crystal state.

The portion 605 that can be considered a single crystal should satisfyin a region the following conditions:

-   -   having substantially no crystal grains;    -   containing hydrogen or halogen elements for neutralization of        point defects at a concentration of 1×10¹⁵ to 1×10²⁰ atoms/cm⁻³;    -   containing carbon or nitrogen atoms at a concentration of 1×10¹⁶        to 5×10¹⁸ atoms/cm⁻³; and    -   containing oxygen atoms at a concentration of 1×10¹⁷ to 5×10¹⁹        atoms/cm⁻³.

Where a metal element for accelerating crystallization of silicon isused as in this embodiment, the film should contain the metal element ata concentration of 1×10¹⁶ to 5×10¹⁹ cm⁻³. This concentration range meansthat if the concentration is higher than this range, the metalcharacteristics appear to deteriorate the semiconductor characteristics,and that if the concentration is lower than this range, the function ofaccelerating crystallization of silicon is not obtained.

As is understood from the above discussions, a silicon portion that isproduced by illumination with laser light and can substantially beconsidered a single crystal is essentially different from an ordinarysingle crystal such as a single crystal wafer.

The film contracts during the crystallization by laser lightillumination, and resulting strain is accumulated more in peripheralportions of the active layer 604.

Further, in general, the thickness of the active layer 604 is in a rangeof from several hundred angstrom to several thousand angstrom andassumes a square with each side being several micrometers to severalhundred micrometers; that is, it is a very thin film. When the crystalgrowth as shown in FIG. 6C proceeds in the active layer 604 that is avery thin film, stain is concentrated in peripheral portions, that is, aportion around the crystal growth end point, and portions beyond whichthe crystal growth does not proceed.

Mainly for the above two reasons, stain is concentrated in theperipheral portions of the active layer 604. The existence of suchportions in the active layer 604 is not preferable because they mayadversely affect operation of a resulting thin-film transistor.Therefore, it is preferable to eliminate the entire periphery of theactive layer 604 by etching.

Thus, an active layer 606 is obtained which can substantially beconsidered a single crystal like the portion 605 and is less influencedby stress.

After the active layer 606 is obtained, a 1,000-Å-thick silicon oxidefilm as a gate insulating film 607 is formed by the plasma CVD so as tocover the active layer 606. A 5,000-Å-thick polycrystalline silicon filmheavily doped with phosphorus (P) is formed thereon by low-pressure CVDand then patterned, to form a gate electrode 608.

Thereafter, a source region 609 and a drain region 611 are formed in aself-aligned manner by implanting phosphorus (P) ions by the plasmadoping or the ion implantation. A region 610 in which impurity ions arenot implanted because of the existence of the gate electrode 608 servingas a mask is defined as a channel forming region. (FIG. 6E)

Then, a 7,000-Å-thick silicon oxide film as an interlayer insulatingfilm 612 is formed by the plasma CVD by using a TEOS gas. After acontact hole is formed, a source electrode 613 and a drain electrode 614are formed with a multi-layer film of titanium and aluminum. A contactelectrode for the gate electrode 608 is formed at the same time (notshown). Finally, a heat treatment is performed for one hour in ahydrogen atmosphere of 350° C. Thus, a thin-film transistor as shown inFIG. 6F is completed.

A plurality of thin-film transistors formed on the glass substrate 601in the above manner had a mobility variation of about ±5%, whichindicates that the crystallization was conducted uniformly.

Further, the respective active layers 606 are substantially very goodsingle crystal layers.

Since the active layer 606 is made of a silicon film that can beconsidered a single crystal, the thin-film transistor of this embodimentexhibits electrical characteristics equivalent to those of a thin-filmsemiconductor that is produced by using a single crystal silicon filmformed by means of an SOI technique or the like.

Embodiment 3

This embodiment uses the laser annealing apparatus of FIG. 1.

FIG. 9 shows an example of an optical system for laser annealing used inthis embodiment.

The optical system of FIG. 9 is composed of the following components. Anexcimer laser beam generating means K (pulse laser beam generatingmeans) generates an excimer laser beam. Beam expanders L and M expandthe excimer laser beam. A compound-eye-like fly-eye lens N expands thelaser beam in a sectional manner. A first cylindrical lens O convergesthe laser beam into a line shape. A second cylindrical lens P improvesthe uniformity of the linear laser beam in its longitudinal direction. Astage S is moved in direction T indicated by an arrow in FIG. 9 in astate that a substrate bearing an illumination object on its surface isplaced thereon.

In FIG. 9, a path-folding mirror Q and a cylindrical lens R allow laserprocessing to be performed on the object placed on the stage S.

In this embodiment, the interval between the laser light source K andthe cylindrical lens L, the interval between the fly-eye lens N and thefirst cylindrical lens O, and the interval between the cylindrical lensP and the illumination surface are set at 230 mm, 650 mm, and 650 mm,respectively (each interval is equal to a sum of focal lengths of therespective lenses concerned). Apparently these intervals may be changedas occasion demands. The cylindrical lens R has a focal length 120 mm.

Type EX 748 of Lumnics Corp. (KrF excimer laser; wavelength: 248 nm;energy gap (Eg): 5.0 eV; pulse width: 25 ns) is used as the light source(i.e., oscillator) of an excimer laser beam to be input to the aboveoptical system. Another example of the light source is type 3000-308 ofLambda Physic Corp. (XeCl excimer laser; wavelength: 308 nm; pulsewidth: 26 ns). Naturally, other excimer lasers and other types of lasermay be used as long as they are of a pulsed oscillation type.

A laser beam as output from the above optical system has a maximumenergy of 800 mJ/shot.

An excimer laser beam as emitted from the light source has a sectionalshape of 20 mm×30 mm. This beam is shaped and expanded by the beamexpanders L and M into a square beam of 30 mm×30 mm.

The reason for shaping the laser beam into a substantially square beamis that the downstream compound-eye-like fly-eye lens N has a generallysquare sectional shape.

By making the sectional shape of a laser beam entering thecompound-eye-like fly-eye lens N similar to that of the latter, theability of the fly-eye lens N can be utilized at the maximum, so thatthe beam can be divided easily and uniformly. Thus, the uniformity of afinally obtained linear laser beam can be improved.

Naturally the laser beam need not always be shaped into a square beam.However, where the beam is somewhat expanded before entering thecompound-eye-like fly-eye lens N, the latter may be one having a largersize, in which case the accuracy of fine processing needed in forming acompound-eye-like fly-eye lens can be lowered.

The compound-eye-like fly-eye lens N shown in FIG. 8 expands, in asectional manner, the laser beam thus shaped and expanded.

As shown in FIG. 8, the sectional shape of the whole compound-eye-likefly-eye lens N is polygonal, and generally square in this embodiment.The compound-eye-like fly-eye lens N is constructed by arranging,regularly and adjacently, a plurality of convex lenses 801 each having apolygonal sectional shape. In this embodiment, convex lenses having asquare sectional shape are arranged adjacently in matrix form.

Each convex lens may assume a sectional shape other than square; forexample, polygonal shapes such as a rectangle, a triangle, and a hexagonare preferable because they can easily be arranged in a regular manner.

A uniform laser beam can be formed by expanding, in a sectional manner,a laser beam with such a compound-eye-like fly-eye lens N.

The laser beam is then converged by the cylindrical lens O into ahorizontally long beam. Finally, a linear laser beam of about 1 mm inwidth and about 120 mm in length is obtained on the illuminationsurface. (The beam width is defined as a half width of its energydensity profile.)

The energy density profile of the resulting linear laser beam in thewidth direction is a quasi-normal distribution.

If the quasi-normal distribution is not favorable for the purpose ofprocessing, the energy density profile can be made closer to a square byusing a slit. In this case, the slit is inserted downstream of thecylindrical lens O, for example, between the cylindrical lens D and themirror Q. The position and the width of the slit may be determined asrequired.

On the other hand, the cylindrical lens P is used to improve theuniformity of the linear beam in the longitudinal direction.

FIG. 11 illustrates the operation of the cylindrical lens P (1103). Thecylindrical lens 1103 has a role of causing laser beams 1101 coming fromthe respective lenses of the fly-eye lens 1102 to reach approximatelythe same position on an illumination surface 1104. In this manner, thelaser beams are combined uniformly on the illumination surface 1104.

It is noted that the optical system of FIG. 9 is not the only one usablein this embodiment, and any optical system can be employed as long as itcan transform a laser beam into a shape required in this embodiment. Forexample, an optical system shown in FIG. 10, which does not include thelenses L and M, can also be used.

FIG. 12 illustrates how laser light illumination is performed by using alinear laser beam. An illumination object 1202 on a substrate isilluminated with a converged linear laser beam 1201 of 120 mm in lengthand 1 mm in width while being scanned in a scanning direction 1203(direction T in FIG. 9) that is perpendicular to the beam longitudinaldirection.

Since the pulsed laser is used, as linear pulse laser beams are moved inthe scanning direction 1203 (indicated by an arrow in FIG. 12) relativeto the illumination object 1202 while the laser is oscillated, they areoverlapped with each other in the scanning direction 1203. As a result,they effect uniform laser annealing on the illumination object 1202.

In illuminating the illumination object 1202 with a linear laser beam,the entire illumination object 1202 can be illuminated uniformly bymoving it in one direction if the beam is longer than the width of theillumination object 1202. This provides a higher processing ability thana case of using a laser beam having a spot shape such as a rectangle.

Even if the beam is shorter than the width of the illumination object1202, the processing will be easier than in the case of a spot beam.However, in this case, it is necessary to move the beam in verticallyand horizontally relative to the illumination object 1202.

The stage on which a substrate is to be placed is controlled by acomputer, and so designed as to move perpendicularly to the longitudinaldirection of the linear laser beam.

If the stage is provided with an additional function of moving in thelongitudinal direction of the beam, even an illumination object that iswider than the beam length can be laser-processed in its entirety.Further, since the stage incorporates a heater in its lower portion, anillumination object can be kept at a given temperature during laserlight illumination.

FIGS. 13A to 13F show a manufacturing process of forming a crystallinesilicon TFT on a glass substrate using the above laser annealingapparatus.

First, a 2,000-Å-thick silicon oxide undercoat film 102 is formed on aglass substrate 101 (this embodiment uses a square Corning 7059 glasssubstrate with each side being 100 mm) and a 500-Å-thick amorphoussilicon film 103 is formed thereon successively by plasma CVD. Then, a10-ppm nickel acetate aqueous solution is applied to the surface of theamorphous silicon film 103, and a nickel acetate layer is formed by spincoating. Nickel serves to accelerate crystallization of the amorphoussilicon film 103. Better results were obtained by adding a surfactant tothe nickel acetate aqueous solution. Since the nickel acetate layer isvery thin, it does not always assume a film form but this will not causeany problems in the subsequent steps. (FIG. 13A)

The amorphous silicon film 103 is crystallized by performing thermalannealing at 550° C. for 4 hours. During this operation, nickel acts ascrystal nuclei and thereby accelerates crystallization of the amorphoussilicon film 103.

It is due to the function of nickel that the crystallization iscompleted at a low temperature of 550° C. (lower than the strain pointof Corning 7059) and in a short period of 4 hours. For details of thisfunction, reference is made of the Japanese Unexamined PatentPublication No. Hei. 6-244104.

Preferable results were obtained when the concentration of the catalystelement was in a range of 1×10¹⁵ to 1×10¹⁹ atoms/cm³. When theconcentration was higher than 1×10¹⁹ atoms/cm³, a resulting silicon filmassumed metal characteristics, that is, semiconductor characteristicsdisappeared.

In this embodiment, the concentration of the catalyst element in aresulting silicon film was 1×10¹⁷ to 5×10¹⁸ atoms/cm³ in terms of aminimum value in the film. More specifically, these concentration valuesof the catalyst element are minimum values in the silicon film obtainedby analysis and measurement by the secondary ion mass spectrometry(STEM).

To improve the crystallinity of the crystalline silicon film thusobtained as well as to lower the degree of nickel segregation in thefilm, laser light emitted from a large-output excimer pulsed laser isapplied to the film. The laser light is shaped into a linear beam of1×120 mm².

The glass substrate 101 is placed on the stage, and illuminated with alaser beam while being moved relative to the laser beam. In thisembodiment, the laser beam illuminating position is fixed while thestage is moved approximately perpendicularly to the laser beamlongitudinal direction (this allows most efficient laser beamprocessing).

The laser beam pulse rate is set at 30 pulses/s and the stage movementspeed is set at 2 mm/s. With this setting, an arbitrary point on theillumination object is illuminated with 15 shots of laser beams. Highlyuniform silicon films were obtained by the laser light illumination whenthis number of shots was set in a range of 2 to 20. The substratetemperature was set at 200° C. during the laser light illumination.

In this embodiment, two-step laser light illumination is employed. Toreduce non-uniformity in laser light illumination effects, it iseffective to perform preliminary illumination/scanning with weak pulselaser light before main illumination/scanning with strong pulse laserlight.

Being highly effective, the two-step illumination greatly improves thecharacteristics of a resulting semiconductor device. In the two-stepillumination, residual amorphous portions are crystallized by the firstillumination and the crystallization of the entire film is acceleratedin the second illumination.

By slowly accelerating the crystallization in the above manner, stripedunevenness on the silicon film caused by the illumination with a linearlaser beam was suppressed to some extent. The laser energy density isset in a range of 100 to 500 mJ/cm²; for example, it is set at 220mJ/cm² in the first illumination and at 365 mJ/cm² in the secondillumination.

Although in the above description of the embodiment only nickel isreferred to as the catalyst element for accelerating crystallization, atleast one of elements including nickel, iron, cobalt, platinum, andpalladium may be used instead of nickel. (FIG. 13B)

A thin-film transistor is formed by using the crystalline silicon filmthus obtained.

First, an island-like silicon region 105 is formed by etching thecrystalline silicon film. A 1,200-Å-thick silicon oxide film 106 as agate insulating film is then deposited by plasma CVD by using TEOS andoxygen material gases. During the deposition of the silicon oxide film106, the substrate temperature is set at 250 to 380° C., for example,300° C. Subsequently, an aluminum film (containing silicon at 0.1 to 2%)was deposited by sputtering at a thickness of 3,000 to 8,000 Å, forinstance, 6,000 Å. A gate electrode 107 is formed by etching thealuminum film thus deposited. (FIG. 13C)

Thereafter, an impurity (boron) is implanted into the silicon region 105by ion doping with the gate electrode 107 used as a mask. The doping gasis diborane (B₂H₆) as diluted with hydrogen to 1 to 10%, for instance,5%. The acceleration voltage is set at 60 to 90 kV, for instance, 65 kV,and the dose is set at 2×10¹⁵ to 5×10¹⁵ atoms/cm², for example, 3×10¹⁵atoms/cm². During the ion doping, the substrate temperature is set atthe room temperature. As a result, P-type impurity regions 108 (source)and 109 (drain) are formed. (FIG. 13D)

To activate implanted boron, laser annealing is performed in the samemanner as described above by using the same laser annealing apparatus.The laser energy density is set at 100 to 350 mJ/cm², for instance, 250mJ/cm². Setting is so made that an arbitrary point on the illuminationobject is illuminated with 2 to 20 shots of laser beams. During thelaser light illumination, the substrate temperature is set at 200° C.Thereafter, thermal annealing is performed at 450° C. for 2 hours in anitrogen atmosphere. Although both laser annealing and thermal annealingare performed in the above process, only one of those may be performed.(FIG. 13E)

Subsequently, a 6,000-Å-thick silicon oxide film 110 as an interlayerinsulating film is formed by plasma CVD, and contact holes are formedthrough it. Electrodes/wiring lines 111 and 112 for the source and drainof the TFT are formed with a metal material such as a multi-layer oftitanium and aluminum. Finally, thermal annealing is performed at 200 to350° C. in a hydrogen atmosphere of 1 atm. (FIG. 13F)

The TFT thus formed has a very high mobility of 100 cm²/Vs or more, andcan sufficiently be used as a TFT that is required to have a highmobility, and is used for a shift register of a liquid crystal display.

According to this embodiment, the life of the laser light source of thelaser annealing apparatus can be elongated by 5 to 10% compared with thecase of using two fly-eye lenses.

Embodiment 4

In this embodiment, a description will be made of a case of forming acrystalline silicon TFT by using an optical system which is different inarrangement from the third embodiment and which produces a linear laserbeam having a trapezoidal energy density profile in the width direction.

A manufacturing process of this embodiment will be hereinafter describedwith reference to FIGS. 13A to 13F, which were also used above todescribe the manufacturing process of the third embodiment.

First, a 2,000-Å-thick silicon oxide undercoat film 102 is formed on aglass substrate 101 (this embodiment uses a 0.7-mm-thick, square Corning1737 glass substrate with each side being 300 mm; alternatively, otherglass materials such as Corning 7059, OA2 and NA45 may be used) and a500-Å-thick amorphous silicon film 103 is formed thereon successively byplasma CVD.

Then, a 10-ppm nickel acetate aqueous solution is applied to the surfaceof the amorphous silicon film 103, and a nickel acetate layer is formedby spin coating. Better results were obtained by adding a surfactant tothe nickel acetate aqueous solution. Since the nickel acetate layer isvery thin, it does not always assume a film form but this will not causeany problems in the subsequent steps. (FIG. 13A)

The amorphous silicon film 103 is crystallized by performing thermalannealing at 550° C. for 4 hours. During this operation, nickel acts ascrystal nuclei and thereby accelerates crystallization of the amorphoussilicon film 103.

It is due to the function of nickel that the crystallization iscompleted at a low temperature of 550° C. (lower than the strain pointof Corning 1737) and in a short period of 4 hours.

Preferable results were obtained when the concentration of the catalystelement was in a range of 1×10¹⁵ to 1×10¹⁹ atoms/cm³. When theconcentration was higher than 1×10¹⁹ atoms/cm³, a resulting silicon filmassumed metal characteristics, that is, semiconductor characteristicsdisappeared.

In this embodiment, the concentration of the catalyst element in aresulting silicon film was 1×10¹⁷ to 5×10¹⁸ atoms/cm³ in terms of aminimum value in the film. More specifically, these concentration valuesof the catalyst element are minimum values in the silicon film obtainedby analysis and measurement by the secondary ion mass spectrometry(STEM).

A crystalline silicon film is obtained in the above manner.

In this state, the glass substrate 101 is so warped that the surfaceformed with the crystalline silicon film is dented, and there is aheight difference of about 300 μm between the center and the peripheriesof the glass substrate 101.

The degree of the warp depends on the size and the type of the glasssubstrate. The warp is about 20 to 200 μm in a square substrate of 100mm by 100 mm, and it may amount to as large as about 1 to 2 mm in asquare substrate of 500 mm by 500 mm.

To improve the crystallinity of the crystalline silicon film obtainedabove, the silicon film is illuminated with laser light emitted from alarge-output excimer pulsed laser.

In this embodiment, the laser annealing apparatus of FIG. 1 is used asin the case of the third embodiment.

In this laser annealing apparatus, an energy variation from one laserbeam pulse to another falls within ˜3% in terms of 3σ. Although a pulsedlaser having a larger energy variation than the above range may be used,the depth of focus of laser beams in the entire illumination step of onescan becomes narrower in such a case. Laser annealing apparatuses withan energy variation larger than ±10% in terms of 3σ are not suitable foruse in this embodiment.

In this embodiment, type 3000-308 of Lambda Physic Corp. (XeCl excimerlaser; wavelength: 308 nm; pulse width: 26 ns) is used as an oscillator.Naturally, other types of pulsed oscillation laser may be used.

A laser beam, which assumes a rectangle of about 3×2 cm² immediatelybefore entering the optical system, is shaped into a long and narrowbeam (i.e., linear beam) of 10 to 30 cm in length and 0.01 to 0.3 cm inwidth by the optical system.

To transform the shape of an emitted laser beam, it is input to anoptical system as shown in FIG. 10. FIG. 10 shows an example of a laserannealing optical system for transforming a laser beam into a linearbeam according to this embodiment. The optical path of this opticalsystem will be described below.

A laser beam that has been emitted from a laser light source k and inputto the optical system first passes through a compound-eye-like fly-eyelens i, which is structured as shown in FIG. 8.

Thereafter, the laser beam passes through a cylindrical convex lens m(first cylindrical lens) and a cylindrical convex lens n (secondcylindrical lens) for improving the uniformity in the longitudinaldirection of a resulting linear beam, reflected by a mirror o, convergedby a cylindrical lens p, and finally applied to an illumination object.

As for optical path lengths, the distance between the laser light sourceand the mirror o is 2,000 mm, and the distance between the mirror o andthe illumination surface is 440 mm. The cylindrical lens p has a focallength of about 100 mm.

A slit is inserted into the laser beam optical path to provide a sharplaser beam energy profile in the width direction at peripheral portionsin that direction. It is preferred that the slit be inserted downstreamof the cylindrical lens n; for example, between the cylindrical lens nand the mirror o, or between the cylindrical lens p and the illuminationobject (see FIG. 10).

Since this optical system uses only one compound-eye-like fly-eye lensand does not use any beam expanders, it is one of configurations mostsuitable for preventing reduction in the energy efficiency of a laserbeam.

Apparently, this embodiment may employ a configuration including beamexpanders as in the case of the optical system of FIG. 9.

A laser beam as output from the optical system, which has been shapedinto a linear beam, has a beam area of 300 mm×0.4 mm on the illuminationsurface. (The beam width is defined as a half width of an illuminationenergy profile.) The energy of a laser beam as output from the opticalsystem is 1,000 mJ/shot at the maximum.

Any optical system may be used as long as it can transform a laser beaminto a beam having a shape that is required by the invention.

As shown in FIG. 12, laser light illumination is performed while alinear laser beam is moved relative to an illumination object. The laserbeam (actually a substrate is moved) is moved approximatelyperpendicularly to the longitudinal direction of the laser beam (i.e.,direction r in FIG. 10).

Next, referring to FIGS. 4A and 4B, a description will be made of theenergy density profile in the width direction of a linear laser beam onthe illumination surface.

In this embodiment, the energy density profile in the width direction ofa linear laser beam on the illumination surface is formed by using aslit. A trapezoidal energy density profile shown in FIG. 4B is usedrather than a commonly used rectangular energy density profile shown inFIG. 4A.

A laser beam having a trapezoidal energy density profile in the widthdirection on the illumination surface has the following advantages overa laser beam having a rectangular energy density profile:

1) a wide depth of focus; and

2) one scanning provides effects equivalent to those obtained by theconventional two-step illumination.

At present, the commonly used energy density profile in the widthdirection of a laser beam is a rectangular profile as shown in FIG. 4A.Although a rectangular laser beam provides a high energy density on theillumination surface, it tends to have a narrow depth of focus;specifically, less than about ±200 μm. Therefore, where the illuminationsurface has asperities or undulation, a rectangular laser beam is likelyto cause a non-uniform distribution of crystallinity as compared with atrapezoidal laser beam. When a plurality of thin-film transistors areformed by using a crystalline silicon film that has such a non-uniformdistribution of crystallinity, they will have non-uniform thresholdvoltages (V_(th)).

On the other hand, the energy density profile shown in FIG. 4B satisfiesboth inequalities 0.5L1≦L2≦L1 and 0.5L1≦L3≦L2 in the width direction ofthe linear laser beam.

A laser beam having such a trapezoidal energy density profile canprovide a wider depth of focus; specifically, about ±400 μm. Therefore,even where the illumination surface has asperities or undulation, atrapezoidal laser beam can provide a more uniform distribution ofcrystallinity than a rectangular laser beam, as well as a sufficientlyhigh energy density for crystallization.

A trapezoidal or triangular energy density profile with a relationshipL2 (L3)>L1 can provide a depth of focus wider than ±400 μm. However, inthis case, focus adjustment is difficult and the energy density is low.Therefore, a resulting silicon film is liable to suffer from aninsufficient degree of crystallinity, in which case a desired mobilityis not obtained.

Thus, where a linear laser beam is applied to an illumination surfacehaving a large variation in height such as a substrate havingasperities, a warp, a distortion, a flexure, or the like, by making theenergy density profile in the width direction a trapezoidal one whichsatisfies the above inequalities and provides a wide depth of focus, thelight beam energy can be imparted to the illumination surface moreuniformly to thereby provide a silicon film having a more uniformdistribution of crystallinity than in the case of using a laser beamhaving the conventional rectangular energy density profile.

In this embodiment, in order to provide a trapezoidal energy densityprofile in the width direction of a laser beam on the illuminationsurface, the position of the lens p is changed vertically (in directions in FIG. 10).

The energy density profile of a laser beam on the illumination surfacecan be changed from a profile close to a rectangular to a profile closeto a trapezoid also by moving the illumination surface vertically (indirection s in FIG. 10) relative to the lens p.

In this embodiment, referring to FIG. 4B, the energy density profile inthe width direction of a linear laser beam is a trapezoidal one in whichL1=0.4 mm and L2=L3=0.25 mm. This profile satisfies the inequalities0.5L1≦L2≦L1 and 0.5L1≦L3≦L1.

The degree of expanse of the bottom portions of this trapezoidal profiledepends on the distance between the final lens of the optical system andthe illumination surface. During laser processing, the distance betweenthe final lens of the optical system and the illumination surface varieswith the height of the illumination surface (object). The degree ofexpanse of the bottom portions of the laser beam trapezoidal profilevaries accordingly. However, if the variation range falls within theranges of the inequalities 0.5L1≦L2≦L1 and 0.5L1≦L3≦L1, the depth offocus is sufficiently wide, enabling uniform laser light processing. Theterm “uniform” as used herein means that the mobility variation of thelaser-light-illuminated film in the substrate area falls within ±10%.

The glass substrate 101 is placed on the stage, and illumination iseffected while the stage is moved at 2.5 mm/s.

The laser light illumination conditions are set such that the laserlight energy density is 100 to 500 mJ/cm² (400 mJ/cm² in thisembodiment) and the pulse rate is 200 pulses/s. The term “energydensity” as used herein means a density of the top base portion (whichhas a maximum value) of a trapezoidal laser beam energy profile.

If laser light illumination is effected under the above conditions, anarbitrary point on the sample is subjected to 32-step illumination. Thatis, since the beam takes 0.4 second to pass through an arbitrary point,that point is illuminated with 32 beam pulses during one scan A siliconfilm having the best crystallinity can be obtained by laser lightillumination of 3 to 100 steps, preferably 10 to 40 steps.

In this embodiment, linear laser beams having a trapezoidal energydensity profile are applied to the substrate while being moved relativeto the substrate in the width direction of the laser beams (i.e.,perpendicularly to the longitudinal direction) with overlaps. Since thelaser beam energy density profile has the sloped portions, an arbitrarypoint on the illumination surface is first illuminated with weak laserbeams, then laser beams whose intensity increases gradually, and thenlaser beams whose intensity decreases gradually. Thus, the illuminatingoperation is completed.

FIG. 5 schematically illustrates this illuminating operation. The laserlight energy gradually increases in the first half of the illuminatingoperation (indicated by character A in FIG. 5) and gradually decreasesin the last half (indicated by character B in FIG. 5).

Therefore, by using a linear laser beam having the above trapezoidalenergy density profile, the variation of energy supplied to theillumination region becomes much gentler than in the case of using alinear laser beam having the conventional rectangular energy densityprofile.

This can produce results that are equivalent to those obtained by thetwo-step illumination in which laser light of a low energy density isapplied first (preliminary illumination) and laser light of a highenergy density is then applied (main illumination).

As a result, this embodiment prevents abrupt phase changes in alaser-light-illuminated silicon film, thereby preventing surfaceroughening and accumulation of internal stress. Thus, a uniformdistribution of crystallinity can be obtained.

The substrate temperature is kept at 500° C. during the laser lightillumination to reduce the rates of increase and decrease of thesubstrate surface temperature due to laser light illumination, tothereby reduce the degradation of the uniformity of the illuminationobject. The substrate temperature is set at a value suitable for laserannealing in a range from the room temperature to the strain point ofthe glass substrate.

No specific atmosphere control is performed in the embodiment, that is,the illumination is performed in the air. Alternatively, theillumination may be performed in a vacuum, in an atmosphere of an inertgas such as argon or helium, or in an atmosphere of hydrogen, nitrogen,or the like. (FIG. 13B)

The crystalline silicon film thus obtained has a uniform distribution ofcrystallinity in spite of the fact that the glass substrate 101 iswarped.

Thereafter, a TFT is formed in the same manner as in the thirdembodiment. In the manufacturing process concerned, the laser annealingapparatus of FIG. 1 is used also in the step of recovering thecrystallinity that is performed after the impurity ion implantation.

Although the glass substrate 101 is warped, resulting TFTs have athreshold voltage with variation of ±5% in the substrate area; that is,the threshold voltages are very uniform. In contrast, where the energydensity profile in the width direction of a linear laser beam is made arectangular one that is commonly used at the present time, thresholdvoltages of TFTs are much different between a central portion andperipheral portions of a substrate. In such a case, a threshold voltagevariation in the substrate area will become ±15% to ±20%.

In addition, according to this embodiment, the life of the laser lightsource of the laser annealing apparatus can be elongated by 5 to 10%,compared with the case of using two fly-eye lenses.

As described above, the invention can improve the productivity incrystallizing a semiconductor coating as well as the degree ofuniformity of crystallinity of the semiconductor coating.

Capable of providing a wide depth of focus, the invention enablesuniform laser annealing to be performed on a film having largeasperities (less than ±400 μm).

The invention can be applied to any type of laser processing used insemiconductor device manufacturing processes. In particular, inmanufacturing TFTs as semiconductor devices, the invention can improvethe uniformity in the substrate area of the threshold voltages of theTFTs, to thereby improve the uniformity of their characteristics.

Further, where the invention is applied to a step of activating animpurity element implanted into the sources and drains of TFTs, theuniformity in the substrate area of the electric field mobility or theon-current of the TFTs can be improved.

Further, by employing the compound-eye-like fly-eye lens in the laserannealing apparatus, the loss of light quantity can be reduced from thecase of using two fly-eye lenses for vertical expansion and horizontalexpansion. As a result, the loss of the laser beam energy is greatlyreduced, that is, the energy efficiency is improved. This makes itpossible to provide superior laser annealing and crystallization of asilicon film, and to elongate the life of a laser light source.

1. A method of manufacturing a semiconductor device comprising the stepsof: forming a semiconductor film over a substrate; and irradiating thesemiconductor film with a laser beam, wherein the laser beam has anenergy profile in a width direction thereof which satisfies inequalities0.5L1≦L2≦L1 and 0.5L1≦L3≦L1 on an illumination surface of thesemiconductor film where assuming that a maximum energy is 1, L1 is afirst beam width of two points having an energy of 0.95 and L1+L2+L3 isa second beam width of two points having an energy of 0.70, L2 and L3occupying two peripheral portions of the first beam width.
 2. The methodaccording to claim 1, wherein the laser beam is a pulse laser beam. 3.The method according to claim 1, wherein the substrate is heated whileirradiating the semiconductor film with the laser beam.
 4. The methodaccording to claim 1, wherein irradiating the semiconductor film isperformed in a vacuum.
 5. The method according to claim 1, whereinirradiating the semiconductor film is performed in an atmosphere of aninert gas.
 6. The method according to claim 1, wherein irradiating thesemiconductor film is performed in an atmosphere of hydrogen ornitrogen.
 7. The method according to claim 1, further comprising a stepof adding a catalyst element to the semiconductor film prior to the stepof irradiating the semiconductor film with the laser beam.
 8. The methodaccording to claim 1, wherein the semiconductor film contains animplanted impurity and the implanted impurity is activated by the stepof irradiating the semiconductor film with the laser beam.
 9. A methodof manufacturing a semiconductor device comprising the steps of: forminga semiconductor film over a substrate; and irradiating the semiconductorfilm with a laser beam so that one point of the semiconductor film isirradiated overlappingly with the laser beam plural times, wherein thelaser beam has an energy profile in a width direction thereof whichsatisfies inequalities 0.5L1≦L2≦L1 and 0.5L1≦L3≦L1 on an illuminationsurface of the semiconductor film where assuming that a maximum energyis 1, L1 is a first beam width of two points having an energy of 0.95and L1+L2+L3 is a second beam width of two points having an energy of0.70, L2 and L3 occupying two peripheral portions of the first beamwidth.
 10. The method according to claim 9, wherein the laser beam is apulse laser beam.
 11. The method according to claim 9, wherein thesubstrate is heated while irradiating the semiconductor film with thelaser beam.
 12. The method according to claim 9, wherein irradiating thesemiconductor film is performed in a vacuum.
 13. The method according toclaim 9, wherein irradiating the semiconductor film is performed in anatmosphere of an inert gas.
 14. The method according to claim 9, whereinirradiating the semiconductor film is performed in an atmosphere ofhydrogen or nitrogen.
 15. The method according to claim 9, wherein theplural times is 3 to 100 times.
 16. The method according to claim 9,further comprising a step of adding a catalyst element to thesemiconductor film prior to the step of irradiating the semiconductorfilm with the laser beam.
 17. The method according to claim 9, whereinthe semiconductor film contains an implanted impurity and the implantedimpurity is activated by the step of irradiating the semiconductor filmwith the laser beam.
 18. A method of manufacturing a semiconductordevice comprising the steps of: forming a semiconductor film over asubstrate; and irradiating the semiconductor film with a laser beam,wherein the laser beam has a trapezoidal energy profile in a widthdirection thereof which satisfies inequalities 0.5L1≦L2≦L1 and0.5L1≦L3≦L1 on an illumination surface of the semiconductor film whereassuming that a maximum energy is 1, L1 is a first beam width of twopoints having an energy of 0.95 and L1+L2+L3 is a second beam width oftwo points having an energy of 0.70, L2 and L3 occupying two peripheralportions of the first beam width.
 19. The method according to claim 18,wherein the laser beam is a pulse laser beam.
 20. The method accordingto claim 18, wherein the substrate is heated while irradiating thesemiconductor film with the laser beam.
 21. The method according toclaim 18, wherein irradiating the semiconductor film is performed in avacuum.
 22. The method according to claim 18, wherein irradiating thesemiconductor film is performed in an atmosphere of an inert gas. 23.The method according to claim 18, wherein irradiating the semiconductorfilm is performed in an atmosphere of hydrogen or nitrogen.
 24. Themethod according to claim 18, further comprising a step of adding acatalyst element to the semiconductor film prior to the step ofirradiating the semiconductor film with the laser beam.
 25. The methodaccording to claim 18, wherein the semiconductor film contains animplanted impurity and the implanted impurity is activated by the stepof irradiating the semiconductor film with the laser beam.
 26. A methodof manufacturing a semiconductor device comprising the steps of: forminga semiconductor film over a substrate; generating a laser beam;expanding and shaping the laser beam with a beam expander; expanding,sectionally, the expanded and shaped laser beam with a fly-eye lens;converging the sectionally expanded laser beam into a linear laser beamwith a first cylindrical lens; improving uniformity of the linear laserbeam in a longitudinal direction thereof with a second cylindrical lens;and irradiating the semiconductor film with the linear laser beam,wherein the linear laser beam has an energy profile in a width directionthereof which satisfies inequalities 0.5L1≦L2≦L1 and 0.5L1≦L3≦L1 on anillumination surface of the semiconductor film where assuming that amaximum energy is 1, L1 is a first beam width of two points having anenergy of 0.95 and L1+L2+L3 is a second beam width of two points havingan energy of 0.70, L2 and L3 occupying two peripheral portions of thefirst beam width.
 27. The method according to claim 26, wherein thelaser beam is a pulse laser beam.
 28. The method according to claim 26,wherein the substrate is heated while irradiating the semiconductor filmwith the laser beam.
 29. The method according to claim 26, whereinirradiating the semiconductor film is performed in a vacuum.
 30. Themethod according to claim 26, wherein irradiating the semiconductor filmis performed in an atmosphere of an inert gas.
 31. The method accordingto claim 26, wherein irradiating the semiconductor film is performed inan atmosphere of hydrogen or nitrogen.
 32. The method according to claim26, further comprising a step of adding a catalyst element to thesemiconductor film prior to the step of irradiating the semiconductorfilm with the laser beam.
 33. The method according to claim 26, whereinthe semiconductor film contains an implanted impurity and the implantedimpurity is activated by the step of irradiating the semiconductor filmwith the laser beam.